As wireless communication technologies develop rapidly, 2nd Generation (2G), 3rd Generation (3G) and 4th Generation (4G) systems co-exist and operate with incumbent systems. However, since all available frequencies are occupied, a new frequency assignment within a few gigahertz is difficult, and limited free frequency bands cause frequency interference between heterogeneous devices in deploying a new wireless communication system. Yet, there are unused spectrums at or above 2 GHz and partially unused TV spectrums at or below 1 GHz. Hence, the Federal Communications Commission (FCC) has conducted a study on real spectrum use of these unused or under-utilized frequencies and sought comments on unlicensed access to unused frequencies through the Notice of Proposed Rule Making (NRPM) in order to increase frequency use efficiency.
For example, there is the Cognitive Radio (CR) system, which senses unused and empty frequencies (or channels) in the licensed band, and uses the unused frequency. Using CR technologies, a CR system can solve the problem of frequency shortage but must not give any inference to primary users in an incumbent system, which pays for and uses the frequency band, during the frequency sharing. That is, considering a communication environment in which the CR system co-exists with the incumbent system (e.g., TV broadcasting system), whenever terminals of an incumbent system want communication service, they can use licensed bands after accessing the channels. However, Customer Premise Equipments (CPEs) of the CR system can use the licensed band of the incumbent system and cause no interference to the incumbent system.
There are two means for the CR system to coexist with the incumbent system. First, it can seek to utilize remaining “available channels” which are not occupied by the incumbent system. Second, it can utilize those channels occupied by the incumbent system, given that the transmission devices in the CR system are far enough away from the incumbent system so that no excessive interference would be caused to the incumbent system. In this invention, we are in particular considering the second method of co-existence.
In the first case, the CR system does not cause interference to the incumbent system when the CR system opportunely uses the empty channels after detects empty channels. For example, when the incumbent system is operating channel n, the CR system uses another channel excluding the channel n, or will not use channel n while the incumbent system uses channel n. Therefore, the CR system gives no interference to the incumbent system. However, in the second case, the incumbent system and the CR system do not intersect and the CR system is far enough away from the incumbent system independently so that CR system and the incumbent system can use channel n simultaneously. But, it is necessary to control power so that a signal of the CR system does not cross a boundary where that of the incumbent system reaches. For example, in an Orthogonal Frequency Division Multiple Access (OFDMA)-based CR system, because multiplexed sub-carriers are allocated to multiple users, the multiple users can use the same OFDM symbol. When each of N user terminals transmits with 1 W power, the incumbent system receives interference of total N*1 W power.